The present invention relates to a monitoring system of repeaters for an optical transmission system, in particular, relates to that system for an optical digital transmission system. The present invention can be applied, for example, to a fault monitoring or a fault localization in a PCM optical transmission system which comprises optical fiber cables for each of upward and downward directions, and a plurality of PCM optical regenerative repeaters. In this case, it is supposed that each upward repeater is located at the same position as that of the corresponding downward repeater.
In the conventional fault monitoring system for the PCM transmission line, interstitial copper pair which are provided around the main transmission line are utilized in order to transmit a repeater selection signal and the result of the fault monitoring to a terminal station. However, in a long distance optical PCM submarine cable system, the presence of said interstitial copper pair makes the structure of the whole cable complicated, and require the additional repeater system for the interstitial copper pair themselves, thus the whole cable system becomes uneconomical. Further, according to a prior repeater monitoring system for PCM transmission lines, a so-called out-service monitor service has been utilized in which the main transmission line is switched to standby or idle, and a particular code trains for the purpose of only the fault location are transmitted through said intervened core cables. However, in an international long distance optical PCM submarine cable system, preventive maintenance which always monitors the operation of the cable system, and provides the basic data for locating the fault repeater, for the immediate recovery of the system fault is important. Accordingly, an in-service monitoring system in which the operation of each repeater during operation can be monitored at terminal stations on land is a more effective system.